Equalizers, also known as transversal filters, are widely-used devices for compensating for distortion in the transmission channel of a digital communication system. In many applications, the equalizer is designed to be "adaptive", i.e., the compensation provided tracks the variations in transmission channel distortion over time. Such tracking is provided by a variety of schemes which adjust the tap-weight coefficients of the equalizer in response to a derived error signal and the equalizer input or output signal. These schemes are often referred to as being "data decision directed" in that the tap-weight coefficient adjustments are affected by the values of the regenerated data.
A persistent problem in the use of equalizers is that the receiver terminal in which the equalizer is disposed can lose synchronization with the incoming data signal. During such times, the receiver terminal is said to be "out of lock" and the data decision directed algorithms may improperly adjust the tap-weight coefficients. This improper adjustment increases the time needed for the receiver terminal to resume proper operation and this additional delay can be unacceptable for certain communication system applications.
Several techniques, such as those disclosed in an article entitled "Self-Recovering Equalization and Carrier Tracking in Two-Dimensional Data Communication Systems" by Dominique N. Godard, IEEE Transactions on Communications, Vol. COM-28, No. 11, November 1980, pages 1867-75, and in a patent application bearing Ser. No. 714,441, and entitled "A Cross-Polarization Canceler/Equalizer", by Gerard J. Foschini, filed May 21, 1985 and assigned to the present assignee, have been devised to provide distortion compensation which is not data decision directed. These techniques, however, require a timing signal which is synchronous with the incoming data signal. Supplying this timing signal when the receiver terminal is out of lock can be difficult, if not impossible. This is especially true in radio systems transmitting a carrier having a particular polarization during periods of severe multipath fading and in radio systems simultaneously transmitting carriers having orthogonal polarizations during periods of even less severe fading. Consequently, a tap-weight coefficient adjustment technique which is asynchronous in operation and which is non-data decision directed would be desirable.